Polymeric nitrogen is a promising candidate for a high-energy-density material.Synthesis of energetic compounds with high chemical stability under ambient conditions is still a challenging problem.Here we report a theoretical study on yttrium nitrides by first principles calculations combined with an effective crystal structure search method.It is found that many yttrium nitrides with high nitrogen content can be formed under relatively moderate pressures.The results indicate that the nitrogen-rich YN5 and YNs compounds are recoverable as metastable high-energy materials under ambient conditions,and can release enormous energies (2.51 kJ.g-1 and 3.18 kJ.g-1) while decomposing to molecular nitrogen and YN.Our findings enrich the family of transition metal nitrides,and open avenues for design and synthesis of novel high-energy-density materials.